Thermal power generation facilities and boiler facilities generate a quantity of carbon dioxide as a result of burning a large amount of fuel, such as coal, heavy oil and the like. From the viewpoint of air pollution or global warming, many countries promote the regulation of large emissions of carbon dioxide (hereinafter abbreviated as “CO2”). As a technique for separating and recovering CO2, a chemical absorption method using an aqueous alkanolamine solution as a CO2 absorbing liquid is widely known. FIG. 3 shows one embodiment of a power generation plant comprising a conventional CO2 chemical absorption system. The power generation plant generally comprises at least boiler 1, denitration device 2, air heater 3, electrical dust collector 4, desulfurization device 5, prescrubber 10, CO2 absorption column 20, regeneration column 40, and reboiler 60. Nitrogen oxides in combustion exhaust gas (e.g., produced from coal combustion) discharged from the boiler 1 are removed in the denitration device 2, and the combustion exhaust gas is then cooled to, for example, 120 to 170° C. by heat exchange with the air heater 3. After the exhaust gas passes through the air heater 3, dust is removed from the exhaust gas by the electrical dust collector 4, and sulfur oxides (SO2) are removed by the desulfurization device 5. About tens of ppm of SO2 may remain in the exhaust gas at the outlet of the desulfurization device 5; thus, in order to prevent deterioration of the CO2 absorbing liquid in the CO2 absorption column 20 by the remaining SO2, the remaining SO2 is reduced to as minimum as possible (e.g., 10 ppm or less) by the prescrubber 10, which is provided as a pretreatment facility in the CO2 chemical absorption system.
The CO2 absorption column 20 comprises at least packed bed 21, absorbing liquid feed part 22, water washing part 24, washing water feed part 25, mist eliminator 26, washing water collector 27, washing water cooler 28, and washing water pump 29. In the packed bed 21, CO2 contained in the exhaust gas is brought into gas-liquid contact with the CO2 absorbing liquid fed from the absorbing liquid feed part 22 in the upper portion of the CO2 absorption column 20, and the CO2 is absorbed by the CO2 absorbing liquid. The heat generated during CO2 absorption raises the temperature of the combustion exhaust gas from which CO2 has been removed. In the water washing part 24, the combustion exhaust gas from which CO2 has been removed is cooled, and mist entrained in the gas is removed. The washing water cooled by the washing water cooler 28 is used circularly by the washing water pump 29. The mist eliminator 26 disposed above the water washing part 24 removes the entrained mist that has not been removed in the water washing part. The combustion exhaust gas processed with the above removal treatment is discharged out of the system as treatment gas 37 (CO2-removal gas).
The absorbing liquid that has absorbed CO2 (also referred to as “CO2-rich liquid”) is extracted by a pump 33 from a liquid storage part in the lower portion of the absorption column 20, heated by a heat exchanger 34, and then sent to the regeneration column 40. In the regeneration column 40, the CO2-rich liquid is fed to a packed bed 41 from a feed part 42. On the other hand, in the bottom of the regeneration column 40, vapor of the absorbing liquid is fed to the packed bed 41 from the reboiler 60 through a vapor feed pipe 65. In the packed bed 41, the rich liquid and the absorbing liquid vapor are brought into gas-liquid contact to desorb CO2 gas from the CO2-rich liquid. Since the desorbed CO2 gas may entrain mist of the absorbing liquid, the mist is removed and the CO2 gas is cooled in a water washing part 43. The entrained mist that has not been removed in the water washing part is removed by a mist eliminator 45 disposed above the water washing part 43. The CO2 gas 46 from which the mist has been removed is discharged from the upper portion of the regeneration column 40. Thereafter, water vapor entrained in the CO2 gas is cooled by a condenser 47, and separated into gas and condensed water (reflux water) by a reflux water drum 48. The CO2 gas is introduced into a CO2-liquefying facility (not shown). The condensed water (reflux water) is fed to a washing water feed part 44 by a drain pump 50.
On the other hand, the CO2 absorbing liquid from which CO2 has been desorbed (also referred to as “lean liquid”) is stored in a liquid collector 51 in the regeneration column. A part of the CO2 absorbing liquid is sent to the reboiler 60 through a reboiler liquid feed pipe 52. The reboiler 60 is provided with a heat exchanger tube, etc., therein. The CO2 absorbing liquid is indirectly heated by water vapor 62 fed through a water vapor feed pipe, thereby generating vapor of the absorbing liquid in the reboiler 60. The absorbing liquid vapor is fed to the regeneration column 40 through the absorbing liquid vapor feed pipe 65 mentioned above. The water vapor used in the reboiler 60 is condensed in the heat exchanger tube, and collected as drain water. The lean liquid stored in the liquid storage part at the bottom of the regeneration column 40 is cooled by the heat exchanger 34 and a cooler 30 through a liquid extraction pipe 66, and then fed to the CO2 absorption column.
In the conventional regeneration column 40, the reflux water returned to the regeneration column 40 from CO2 separation drum (reflux water drum) 48 is brought into direct contact with the gas in the water washing part 43, and then added dropwise to the packed bed 41 to condense a part of the absorbing liquid vapor fed from the reboiler 60. This is uneconomical in that the reflux water, which is not essentially necessary to be heated, is unnecessarily heated.